The invention relates to a method for measuring the shape of a wavefront of an optical radiation field that is generated by an active radiation source, particularly a method for determining at least one radius of curvature of the wavefront. Furthermore, the invention relates to a measuring apparatus for measuring the shape of a wavefront, particularly to determine at least one radius of curvature of the wavefront, of an active radiation source. Applications of the invention exist particularly in the characterization of radiation sources, particularly laser sources, for example, for data transmission purposes.
A radiation source for optical data transmission comprises, for example, a solid-state laser which is coupled with an optical fiber. A free end of the optical fiber forms a point light source, whose light is converted into a collimated radiation field using a collimation optics. Greater requirements are placed on the collimation of the radiation field for data transmission across large distances, for example, in outer space. The wavefront radius of curvature of the radiation field is typically required to be greater than 150 m, particularly greater than 250 m. Such large radii of curvature represent a challenge for measurement technology. For example, with the radiation field having a diameter of 12 mm and a radius of curvature of 250 m, the pitch of the wavefront is only 72 nm. Generally, it is difficult to record such a small curvature by optical measurement due to the high noise component in the measuring signal of a wavefront sensor. Therefore, the measuring ranges of commercially available wavefront sensors are typically limited to maximum measurable radii of curvature in the range of 50 m to 150 m.
In a standard method for measuring wavefronts, a Shack-Hartmann type wavefront sensor is used, in which the wavefront is imaged with a microarray of optical lenses onto a spatially-resolving optical sensor (see for example WO 01/028411 A1 or EP 0 921 382 A2). This enables the total wavefront to be recorded with one single measurement. However, the disadvantage is that the Shack-Hartmann wavefront sensor is limited to radii of curvature less than 100 m. Thus, a practical application in testing radiation sources for optical data transmission is excluded.
In another type of wavefront sensor, the lens microarray is replaced by a movable pinhole diaphragm. The pinhole diaphragm lets through a part of the radiation field to be investigated (subaperture), which is imaged onto a spatially-resolving optical sensor by the focusing optics. In the case of an ideally collimated radiation field, each part of the wavefront would be imaged onto the optical axis of the focusing optics and in the center of the optical sensor. Through the curvature of the real radiation field, abaxial parts of the wavefront are imaged on a position with a lateral deviation from the optical axis of the focusing optics (deviation position, lateral position). The radius of curvature can be calculated for each part of the wavefront from the deviation position. With a movement of the pinhole diaphragm perpendicular to the optical axis of the radiation field to be investigated, different parts of the wavefront are recorded successively with the result that the total wavefront can be characterized (see for example DE 40 03 698 A1 or DE 40 03 699 A1).
In conventional wavefront sensors with the movable pinhole diaphragm, a limitation was found in that the error in the radius of curvature determined for the subapertures of the wavefront increases with increasing distance from the optical axis of the radiation field. This problem is particularly critical when characterizing radiation sources for optical data transmission, said sources being distinguished by a relatively large aperture in the radiation field (for example, 10 mm to 20 mm). A further problem occurs when using a pinhole diaphragm with several diaphragm openings for multiplex measurements according to DE 40 03 698 A1. Several parts of the wavefront are recorded simultaneously by the optical sensor, which can, however, lead to superposition of diffraction phenomena on adjacent diaphragm openings and to signal corruption by crosstalk. This corruption also particularly affects the measurement of radii of curvature exceeding 150 m. Therefore, the use of conventional wavefront sensors in measuring large radii of curvature is limited.
The problems mentioned do not only occur when characterizing radiation sources for data transmission but also in other optical components as often stipulated, for example, in collimators with stringent requirements on the parallelism of the emitted beams, for example, in collimators, particularly for high-resolution MTF measurements.